Q: Why do we need to have a heartworm test every year? A: The annual test ensures that heartworms are not present in your animal. Having heartworms can be fatal to a pet in a short period of time, so the sooner they are detected, the sooner they may be exterminated. So why test if the pet has been on Heartgard since the previous test? Because Heartgard guarantees that it will pay for treatment of any protected animal that develops heartworms. However, the guarantee is only in effect if the animal receives an annual heartworm test.